The invention relates to a device for providing a value indicating the position of an object which can occupy at least three consecutive positions, comprising an arrangement with several terminals each of which corresponding to one of the positions, the application of a DC voltage at each instant on one and only one of the said terminals indicating the presence of an object in the corresponding position.
Such devices are used for example for transmitting the position of a moving body to a display unit, for example the position of a vane, or for transmitting the position of a control device, for example a rotary knob, or a hand operated multi-contact slider, to a device to be controlled.
Known electromechanical devices are constituted of mechanical displacement detectors (cams, ratchets, Maltese crosses, etc. . . . ), operating electrical contacts. These devices are generally bulky, not very reliable and only accept low displacement speeds.
There are also optical encoders formed from a light source (generally light emitting diodes), photodetectors and a focusing system. These optical sensors, without contacts, are very widely used. They are rather bulky and cannot be used in equipment in which the operating temperature limits are very severe.
Phase shifting electromechanical devices are based on systems (sine-cosine potentiometers, resolvers, etc.) delivering signals which are phase-shifted as a function of the direction of rotation. These special components are bulky, of limited lifetime and are relatively costly.
A device using temporal analysis by microprocessor could be envisaged. But the embodiment of this device would be complex, taking account of the large number of states to be tested, for transition times in the order of a few milliseconds.
Encoding devices are normally of two types: either "short-circuiting", i.e. when passing from one position to the next, two positions are momentarily indicated simultaneously, or "non-short-circuiting" in which case there is a break of continuity between two positions. In order to avoid this, the document U.S. Pat. No. 2,902,685 recommends the use of displays with a trigger voltage higher than their operating voltage, in series with a resistor which is common to all of the displays such that the switching on of one of them makes the voltage drop and prevents the triggering of a second one, which prevents the display of two positions at the same time with a short-circuiting type encoder. Similar results could easily be obtained with threshold semiconductor flip-flops.
Document DE-P-2,247,777 describes a device which can obtain the same result using non-short-circuiting sensors: R/S flip-flops record a position and store it during the time of transition from one position to the next.
With these two solutions, a voltage on a terminal which corresponds to one position can be applied at each instant to one and only one of the said terminals. The words "at each instant" means that at no time can the situation in which no voltage would be applied to at least one of the terminals arise: this of course ignores the intrinsic transition times of the electronic circuits used.